Reduction of metal-oxygen compounds, such as metal-oxides, for instance iron oxides, has been performed in large-scale reduction furnaces. For the reduction of iron-oxygen compounds, the blast furnace has been the workhorse for the production of pig iron from iron ore for over a century. The primary reductant and source of chemical energy in these blast furnaces is coke. Coke is produced by baking coal in the absence of oxygen in order to remove the volatile hydrocarbons and to give the coke the critical properties for stable blast furnace operation.
Coke making is problematic from an environmental perspective as many of the volatile hydrocarbons are hazardous. Also not all types of coal are suitable for coke making. Moreover, demand has decreased for the by-products of coke making.
Therefore, decreasing the coke rate and the over-all fuel rate of the blast furnace has been a major focus of recent developments. Also new technologies to circumvent the blast furnace process, such as direct reduction of iron ore, have been developed.
Direct reduction involves the production of iron by reduction of iron ore with a reducing agent, which can be a solid reducing agent or a gaseous reducing agent. The solid reducing agents may be coal of any size, instead of coke. Examples of gaseous reducing agents are natural gas and carbon monoxide. Ores for direct reduction have to meet stringent specifications with high percentage of Fe and low content of unwanted elements.
Direct reduction of iron ore may produce a solid direct-reduced iron product or, at high operating temperatures or in combination with a smelting device, a liquid product.
The product of a direct reduction process may be discharged into a second reactor for melting and optional further refining, or cooled and stored for later use.
At present, dust and sludge from an integrated steelworks is recycled as raw material in the ore preparation stage. These waste materials, often referred to as ‘fines’, may contain iron-containing compounds such as iron oxides. However, due to the high content of metals such as zinc in these fines, the accumulation of such elements, and the limitations of the amount of these metals for charging into a blast furnace, these waste materials have often to be recycled in an other way or disposed of, resulting in additional costs or burdening of the environment.
A known process for reduction of iron ore is based on the direct reaction of coal and lump iron ore or pellets in a rotary kiln. Another known process is based on the reduction of composite pellets containing iron oxide and carbon from for example coal, coke or charcoal in a rotary hearth furnace. The off-gases from the reduction reaction can be post-combusted in the furnace to provide a portion of the heat required for the process. Another known process involves direct reduction of fine iron ore in a fluidised bed reactor.
A major disadvantage of these known reduction processes is that they operate at high temperatures. For example, the rotary hearth process operates at temperatures of about 1250° C. If these processes are based upon the use of coal, a further disadvantage is the development of large volumes of carbon monoxide, hydrogen and complex and hazardous hydrocarbons. Condensation of these hydrocarbons must be avoided which requires removal or post-combustion of the off-gases while re-oxidation of the metal must be prevented. Also, because of the high operating temperatures and consequent heat losses, and the generation of large amounts of carbon monoxide, the energy efficiency of the direct reduction processes is generally poor resulting in a high carbon consumption rate. The high operating temperatures also result in the formation of significant amounts of harmful nitrogen-oxygen compounds (NOx-gases). Furthermore, direct reduction technologies based on the use of coal have to deal with higher levels of sulphur because of the presence of sulphur in the coal.
GB-A-1471544 describes a process of direct reduction of iron ore, in which iron oxide, such as magnetite, is mixed with a nucleating agent in the form of ferric chloride and coal is mixed with an activator also in the form of ferric chloride. These two mixtures are thoroughly mixed together and formed into balls. The balls are purged with cold nitrogen, then heated slowly by heated nitrogen to 1050° C., held for 30 minutes then cooled in cold nitrogen. CO2 is formed initially from the reaction of carbon with the oxide. The activating agent promotes the reduction of the CO2 by the carbon to form CO. The nucleating agent (iron from the ferric chloride, aids adsorption of CO at the oxide surface to accelerate the reduction of the oxide by the CO.
U.S. Pat. No. 3,979,206 describes reduction of MgO with carbon at 1000-2000° C. in the presence of iron, cobalt, nickel, chromium or manganese. Fe powder, MgO powder and C powder were heated in a vacuum furnace. Mg vapour is recovered. The Fe is said to act as a catalyst, permitting lowering of the reaction temperature.